The Optical Time Domain Reflectometry (OTDR) technique is well known for characterizing a fiber longitudinally or for interrogating an array of sensors.
This technique consists in injecting into the fiber under test a light pulse with a width of a few tens of nanoseconds at a repetition frequency in the order of 1 kHz. The reflected light is recorded as a function of time; there may be two sources of reflections, either diffusion intrinsic to the vitreous material (Rayleigh backscatter) or the presence of reflective defects. This method can be used to locate and quantify heterogeneities and deformations of the guide.
The same phenomenon can be used to interrogate an array of sensitive points distributed along the fiber. Sensors of this kind can be used to measure pressure forces. They are based on determining losses induced as a result of microcurvature.
The fiber can be positioned between two grooved plates, as described in U.S. Pat. No. 4,463,254.
Another option, as employed in HERGALITE sensors manufactured by HERGA Electric Limited, is to apply periodic microcurvature to the fiber.
European patent application 0238 440 of Mar. 3, 1987 proposes to integrate discontinuities into the protective coating of the fiber.
None of the above structures can be produced continuously while the fiber is being drawn.
An object of the present invention is to avoid this drawback and to propose a process that is much more economical than prior art processes.